Steel pipe used in oil wells to seal off fluids from the bore hole and to prevent the walls of the hole form sloughing off or caving. API casing sizes range from 4 1/2 in. OD to 20 in. OD inclusive. (RP 5A5)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}